The responses of the two cyclic nucleotides, adenosine 3',5'-cyclic monophosphate (cyclic AMP) and guanosine 3',5'-cyclic monophosphate (cyclic GMP) to experimentally induce alterations in brain metabolism were investigated in vivo. Cyclic GMP in the cerebellum increased following electroconvulsive shock (ECS) and after the administration of either CNS Stimulants or convulsants. The levels of cyclic GMP were decreased following the injection of a number of CNS depressants and anticonvulsants. While cyclic AMP also increased following ECS, this cyclic nucleotide is pharmacologically unresponsive. Most of the anticonvulsants tested to date decreased the cyclic GMP levels by greater than 50 percent. The relationship of cerebellar cyclic GMP to both electrically and chemically-induced seizures is currently being investigated.